Matrix elements for exclusive decays of heavy flavour hadrons are investigated in the framework of Bethe-Salpeter approach, incorporating heavy quark flavor and spin symmetries. The Bethe-Salpeter amplitude consists of a spin-parity projector representing the Lorentz property of the hadron and a wave function of the relative motion. It is the former that determines the general covariant structure of hadronic matrix elements, whilst the latter always enters in the guise of form factors. Besides demonstrating the equivalence of several methods of the heavy quark effective theory with Bethe-Salpeter approach in the decays between heavy flavoured hadrons, we establish relations between form factors in weak decays of heavy hadrons into light flavoured resonances of arbitrary spin. Phenomenological consequences of our results on semileptonic and rare dilepton decays of charmed and bottomed hadrons are discussed.